A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of equivalent output power. Such higher temperatures may increase nitrogen-oxide (NOX) emissions and cause accelerated materials ageing in the engine and associated exhaust system. Exhaust-gas recirculation (EGR) is one approach for combating these effects. EGR strategies reduce the oxygen content of the intake air charge by diluting it with exhaust. When the diluted air-exhaust mixture is used in place of ordinary air to support combustion in the engine, lower combustion and exhaust temperatures result. EGR also improves fuel economy in gasoline engines by reducing throttling losses and heat rejection.
In a boosted engine system equipped with a turbocharger compressor and a turbine, exhaust may be recirculated through a high pressure (HP) EGR loop or a low-pressure (LP) EGR loop. In the HP EGR loop, the exhaust is drawn from upstream of the turbine and is mixed with intake air downstream of the compressor. In the LP EGR loop, the exhaust is drawn from downstream of the turbine and is mixed with intake air upstream of the compressor. HP and LP EGR strategies achieve optimum efficacy in different regions of the engine load-speed map. For example, on boosted gasoline engines running stoichiometric air-to-fuel ratios, HP EGR is desirable at low loads, where intake vacuum provides ample flow potential; LP EGR is desirable at higher loads, where the LP EGR loop provides the greater flow potential. Various other tradeoffs between the two strategies exist as well, both for gasoline and diesel engines. Such complementarity has motivated engine designers to consider redundant EGR systems having both an HP EGR loop and an LP EGR loop.
In boosted diesel engines especially, the EGR flow rates needed to keep NOX emissions within acceptable limits are quite high. This can result in a number of issues. First, high levels of intake-air dilution may cause poor combustion stability at lower engine loads, resulting in increased hydrocarbon (HC) and carbon-monoxide (CO) emissions. Second, at high HP EGR flow rates, mass flow through the compressor is significantly reduced, making the compressor prone to surge. To address the first issue, uncooled (i.e., by-passed) HP EGR may be mixed into the intake air, for increased manifold air temperature (MAT). To address the second issue, LP EGR may be used. LP EGR provides increased flow through the compressor, protecting against surge, but is cooled on flowing through the charge-air cooler. In short, to achieve high MAT, exhaust may be routed through an uncooled HP EGR system, but that may cause surge. To avoid surge, the exhaust may be routed through a well-cooled LP EGR system, but this will increase HC and CO emissions. Meanwhile, supplementary or alternative use of cooled HP EGR may introduce further issues. For example, excessive fouling of an EGR cooler can occur when cooled HP EGR flow rate is allowed to fall below a suitable level.
At higher engine loads and speeds, still other issues arise. Flowing exhaust through an LP EGR loop increases the overall flow rate through the compressor, which may cause an over-speed condition and reduced compressor efficiency. This condition, if excessive, could lead to compressor choke. Flowing exhaust through an LP EGR loop also increases the gas temperature at the compressor inlet, which, in turn, increases compressor outlet temperature. On the other hand, uncooled HP EGR at high loads may cause excessive MAT, reducing engine power and/or causing an excessive outflow of smoke from the engine. These conditions may also make the compressor prone to surge.
Other, quite different issues arise during transient conditions. LP EGR, while improving turbocharger efficiency at lower engine loads, takes longer to purge from the intake system. This is because the purge volume includes not only the intake manifold, but everything from the intake manifold to the inlet of the compressor. HP EGR, while more easily purged from the intake manifold, reduces mass flow through the turbine. This reduces turbocharger speed, which increases lag. Thus, both EGR strategies can potentially degrade the engine's ability to respond to load and speed transients.
Further issues arise during catalyst warm up—another transient condition. Being very well cooled, LP EGR does not enable exhaust temperatures as high as uncooled HP EGR. Thus, in some cases catalyst light-off may be unacceptably delayed when running LP EGR. Uncooled HP EGR, on the other hand, enables higher exhaust temperatures but provides significantly reduced mass flow through the exhaust system. This can reduce the thermal energy in the exhaust and may also reduce the heat transfer coefficient.
The inventors herein have recognized these issues and the interrelationships between them, and have devised a series of approaches to address them. Therefore, one embodiment of this disclosure provides a method for charging an intake manifold of an engine. The method comprises adjusting an LP EGR flow rate and an uncooled HP EGR flow rate within first limits to maintain a target dilution level in the intake manifold at steady-state. The method further comprises adjusting the LP EGR and uncooled HP EGR rates within second limits, different from the first, to maintain the target dilution level in the intake manifold during transient conditions. In this manner, HP and LP EGR may be controlled coordinately during steady state and transient conditions. By dynamically changing the values that bracket the HP and LP EGR flow rates depending on conditions, engine longevity, fuel economy, and emissions control can be improved while enabling rapid response to transients.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.